1. Field of the Invention
The present invention relates generally to a secondary lithium battery, and particularly to a secondary lithium battery containing an organic electrolyte.
2. Prior Art
The lithium battery has a high standard electrode potential, i.e. -(minus)3.03 volts based on the normal hydrogen electrode, and thus has an extremely high cell reducing power. In addition, since the atomic weight of lithium is so small as 6.941, the capacity of the cell per unit weight is so large as 3.86 Ah/g. Thus, many researches and studies have been made on batteries wherein lithium is used as the anode material, such a cell or battery being referred to as lithium battery throughout the specification and claims of this invention. As the fruits of the researches, lithium batteries having cathode active materials made of MnO.sub.2 (manganese dioxide) or (CF).sub.n (polyfluorocarbon) have been commercially available. However, such commercial products are primary batteries, and rechargeable lithium batteries for practical uses have not yet been developed. By the provision of a lithium battery, which can be recharged while retaining its favourable high eneregy density discharge characteristics, a lithium battery having exceedingly superior features over those of the conventional batteries could be realized with attendant effect that such a battery may be applied for portable electronic instruments.
It is desirous in view of the practical applications that a nonaqueous electrolyte is used in a lithium battery which is operated in room temperature. However, the conductivity of such an electrolyte is, in general, lower than those of the aqueous electrolytes used in the prior art batteries by one or two ciphers. Under these circumstances, it has been proposed to use electrolytes having high conductivity for applying those for primary lithium batteries. Examples of such electrolytes known in the art are those prepared by admixing ethers to propylene carbonate, ethylene carbonate, .gamma.-butyrolactone or the like and disclosed by U.S. Pat. No. 4,129,691 (1978), and the specifications of Japanese Patent Application Nos. 38030/1976 and 40529/1976. On the other hand, U.S. Pat. No. 4,056,663 (1977) discloses an electrolyte composed of ethylene carbonate and an ester.
On the other hand, it is required that the charge-discharge efficiency of lithium is high in a secondary lithium battery. Lithium reacts with organic solvents thermodynamically to form films on the surface of lithium. The charge-discharge efficiency of the secondary lithium battery is seriously affected by the rate of formation of the film and the properties thereof. In other words, in a secondary lithium battery, the charge-discharge efficiency of lithium has no connection with the conductivity of the used organic solvent, and in fact the charge-discharge efficiencies of lithium in almost all electrolytes practically used in prior art primary batteries are low and thus cannot be used as the electrolytes in a secondary battery. It has already been proposed to use, as the electrolyte in secondary batteries, LiAsF.sub.6 /2-methyltetrahydrofuran (U.S. Pat. No. 4,118,550) and LiClO.sub.4 /1,3-dioxolan (U.S. Pat. No. 4,086,403). The charge-discharge efficiency is increased particularly by the use of 2-methyltetrahydrofuran. This is because a lithium surface film different from the surface film formed by pure 2-methyltetrahydrofuran, when a lithium surface film is compared with that formed in the presence of 2-methylfuran contained in the 2-methyltetrahydrofuran as an impurity. (See J. Electrochem. Soc., vol. 131, 2197 (1984).) It is also reported that the charge-discharge efficiency may be improved by propylene carbonate (Electrochim. Acta, vol. 22, 75 (1977), methyl acetate (Electrochim. Acta, vol. 22, 85 (1977) or tetrahydrofuran (J. Electrochem., Soc., vol. 125, 1371 (1978) in case where water is added. It has been further proposed to use an ethylene carbonate system mixed with an ether as the electrolyte having relatively high charge-discharge efficiency. (See Electrochim. Acta, vol. 30, 1715 (1985) and the specification of Japanese Patent Application No. 206573/1982.) However, these known references neither explicitly disclose the influences of impurities contained in ethylene carbonate/ether system mixed solvents. Nor is there any trial to improve the composition of the lithium surface film.